In the manufacture of semiconductor devices, ion implantation is used to dope semiconductors with impurities. Ion implantation systems are often utilized to dope a workpiece, such as a silicon or germanium wafer, with ions from an ion beam, in order to either produce n- or p-type material doping, or to form passivation layers during fabrication of an integrated circuit. When used for doping semiconductor wafers, the ion implantation system injects a selected ion species into the workpiece to produce desired electrical characteristics in the bulk material of the workpiece. Implanting ions generated from source materials such as antimony, arsenic, or phosphorus, for example, results in an “n-type” material characteristics, whereas a “p-type” material characteristics results from ions generated with source materials such as boron, gallium, or indium.
Typical ion implantation systems include an ion source for generating electrically charged ions from ionizable source materials. The generated ions are formed into a high speed ion beam utilizing a strong electric field to draw ions from the ion source and direct the ions along a predetermined beam path to an implantation end station that allows the workpiece to be transported in the path of the beam. The ion implanter may include beam forming and shaping structures extending between the ion source and the end station. The beam forming and shaping structures maintain the ion beam and bound an elongated interior cavity or passageway through which the ion beam passes en route to the end station. During operation, this passageway is typically evacuated in order to reduce the probability of ions being deflected from the predetermined beam path as a result of collisions with gas molecules.
It is common for the workpiece being implanted in the ion implantation system to be a semiconductor wafer having a size much larger than the size of ion beam. In most ion implantation applications, the goal of the implantation is to deliver a precisely-controlled amount of a dopant uniformly over the entire area of the surface of the workpiece or wafer. In order to achieve the uniformity of doping utilizing an ion beam having a size significantly smaller than the workpiece area, a widely used technology is a so-called hybrid scan system, where a small-sized ion beam is swept or scanned back and forth rapidly in one direction, and the workpiece is mechanically moved along the orthogonal direction of the scanned ion beam.
Alternatively, ribbon beam systems are known, which provide a lengthwise ion beam from the ion source, wherein the beam is allowed to further diverge as it travels toward the workpiece, thereby dispersing ions across a full width of the workpiece as the workpiece is mechanically moved along the orthogonal direction of the lengthwise ion beam. In yet another alternative, so-called “pencil beam” systems are known, wherein the ion beam is presented to the workpiece in the form of a spot while the workpiece is scanned in two dimensions, thereby “painting” the entire wafer with ions from the pencil beam.
A dosage of ions implanted into a workpiece by an ion beam has been conventionally controlled by variations in scan speed (e.g., a speed of the workpiece with respect to the ion beam, or vice-versa). For example, U.S. Pat. No. 4,922,106 to Berrian et al. discloses an ion implantation system wherein an ion beam is scanned and traverses over a workpiece in a controlled manner to attain a selected beam current and corresponding ion dose on the workpiece. A further feature of Berrian et al. is a method and apparatus for sensing the ion beam incident at the workpiece and controlling the exposure of the ion beam to the workpiece in order to attain a selected ion dosage at the workpiece, with the specific object of obtaining a highly uniform dosage over the entire surface of the workpiece.
In more refined integrated circuit manufacturing techniques, control of the implant process by implanting ions in a non-uniform manner can be advantageous for creating multiple regions on the workpiece that are implanted with different dose amounts. For example, U.S. Pat. No. 6,750,462 to Iwasawa et al. discloses an ion implanting method and apparatus wherein a plurality of ion implantation steps are executed by changing a driving speed of the workpiece. Further, a rotating step is provided for rotating the workpiece around its center by a prescribed angle between respective implanting steps while the ion beam is not applied to the workpiece in order to provide an implanted workpiece having a selectively controlled ion dosage across the surface thereof.
Typically, it is desirable to maintain a uniform energy distribution of ions implanted across the surface of a workpiece. However, U.S. Pat. No. 7,576,339 to Rouh et al., discloses an implantation system having an energy control feature for controlling the ion implantation energy of the ion beam based on a region of the wafer being implanted. As disclosed by Rouh et al., the distribution of ion implantation energy into a wafer is discretely (finitely) varied according to discrete or finite regions on the wafer, such that ions are implanted into a first region at a relatively high implantation energy, and ions are implanted into a second region at a relatively low implantation energy. In alternative embodiments of Rouh et al., a first discrete region of the wafer is implanted with a discrete, low implantation energy, a second discrete region is implanted with a discrete, high implantation energy, and a third region is again implanted with the low implantation energy. Again, the ion implantation energy is discretely varied based on discrete regions of the wafer.